1. Field of the Invention
The present invention relates to an object information acquiring apparatus and a control method thereof, and more particularly to a technique to remove an artifact caused by the multiple reflection of an acoustic wave.
2. Description of the Related Art
In medical fields, a technique called “photoacoustic tomography” (PAT) has been proposed in recent years as a technique that can image inside an organism non-invasively, and development thereof is progressing. Photoacoustic tomography is a technique to image an internal tissue that generates an acoustic wave by irradiating pulsed light generated from a light source onto an object, and utilizing a photoacoustic effect where an acoustic wave (typically an ultrasound wave) is generated by absorption of the light which propagated and diffused inside the object. By receiving the acoustic wave generated inside the object using an acoustic wave probe and mathematically analyzing and reconstructing the acquired signals, information related to the optical characteristic values inside the object can be visualized. This apparatus is called a “photoacoustic measurement apparatus” or the like.
In an apparatus that acquires information inside an object using an acoustic wave, as with the photoacoustic measurement apparatus, acoustic impedance matching between the object and the acoustic wave probe on the acoustic wave propagation path is required. If there is an area where the acoustic impedance does not match between the object and the acoustic wave probe, the acoustic wave is reflected by this area, and an image cannot be reconstructed normally.
Actually however a reflection layer often exists between the acoustic wave probe and the object, and it is difficult to perfectly match the acoustic impedance on an acoustic wave propagation path. The reflection layer is, for example, a holding plate used for securing an object. Such a holding plate has an acoustic impedance that is different from that of the organism, hence the acoustic wave is reflected by the interface therebetween. The reflected acoustic wave is also reflected by the interface on the opposite side, and attenuates while repeating the reflection between the probe and the object. In other words, the acoustic wave repeatedly enters the probe and appears as an incorrect image in the acoustic wave transmission direction. This false image is called an “artifact”. An artifact appears with a stronger intensity as the reflection becomes closer to the specular reflection. This problem occurs not only to photoacoustic measurement apparatuses but also to apparatuses which acquire information inside an object by an acoustic wave, such as an ultrasound measurement apparatus using an ultrasound wave echo.
Techniques to remove an artifact generated by the multiple reflection of an acoustic wave includes the following. For example, Japanese Patent Application Laid-open No. 2009-82450 discloses an imaging apparatus that extracts a signal reflected by an interface of a holding plate, and deletes this signal from the acquired image so as to remove the reflection signal.